Stabilization of Metallic Nanowire Meshes Via Encapsulation

ABSTRACT

Techniques for mechanically stabilizing metallic nanowire meshes using encapsulation are provided. In one aspect, a method for forming a mechanically-stabilized metallic nanowire mesh is provided which includes the steps of: forming the metallic nanowire mesh on a substrate; and coating the metallic nanowire mesh with a metal oxide that encapsulates the metallic nanowire mesh to mechanically-stabilize the metallic nanowire mesh which permits the metallic nanowire mesh to remain conductive at temperatures greater than or equal to about 600° C. A mechanically-stabilized metallic nanowire mesh is also provided.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 14/841,944filed on Sep. 1, 2015, the disclosure of which is incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to metallic nanowire meshes, and moreparticularly, to techniques for mechanically stabilizing metallicnanowire meshes against heating using encapsulation.

BACKGROUND OF THE INVENTION

Many semiconductor applications call for transparent electrodes. Commonelectrode materials include transparent conductors such as In₂O₃:Sn(ITO) and ZnO:Al (AZO). These materials can however absorb UV light andfilter out wavelengths used by some devices. In addition to beingtransparent to UV light, for some applications the electrode also needsto be mechanically robust and not degrade at high temperatures. Very fewmaterials exist which are both UV-transparent and mechanically stable.

Nanowire meshes offer excellent conductivity and allow reasonabletransmission of light at all wavelengths. Metallic nanowires, such assilver (Ag) nanowires, are commercially available and inexpensive. Thus,metallic nanowire meshes, such as Ag nanowires meshes are a viablecandidate for transparent electrode materials. However, metallicnanowires undergo microstructural changes at temperatures as low as 400°C., after which the films cease to be conductive.

Therefore metallic (e.g., Ag) nanowire meshes that are mechanicallystable and remain interconnected at high temperatures would bedesirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for mechanically stabilizingmetallic nanowire meshes using encapsulation. In one aspect of theinvention, a method for forming a mechanically-stabilized metallicnanowire mesh is provided. The method includes the steps of: forming themetallic nanowire mesh on a substrate; and coating the metallic nanowiremesh with a metal oxide that encapsulates the metallic nanowire mesh tomechanically-stabilize the metallic nanowire mesh which permits themetallic nanowire mesh to remain conductive at temperatures greater thanor equal to about 600° C.

In another aspect of the invention, a mechanically-stabilized metallicnanowire mesh is provided. The mechanically-stabilized metallic nanowiremesh includes: a metallic nanowire mesh on a substrate; and a metaloxide coating on the metallic nanowire mesh that encapsulates themetallic nanowire mesh to mechanically-stabilize the metallic nanowiremesh which permits the metallic nanowire mesh to remain conductive attemperatures greater than or equal to about 600° C.

In yet another aspect of the invention, another method for forming amechanically-stabilized metallic nanowire mesh is provided. The methodincludes the steps of: forming the metallic nanowire mesh on asubstrate; electroplating a metal onto the metallic nanowire mesh so asto fuse individual metallic nanowires in the metallic nanowire meshtogether; and coating the metallic nanowire mesh with a metal oxide thatencapsulates the metallic nanowire mesh to mechanically-stabilize themetallic nanowire mesh which permits the metallic nanowire mesh toremain conductive at temperatures greater than or equal to about 600° C.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming amechanically-stabilized metallic nanowire mesh according to anembodiment of the present invention;

FIG. 2 is a scanning electron micrograph (SEM) image of a metallicnanowire mesh according to an embodiment of the present invention;

FIG. 3 is a SEM image illustrating the result of subjecting a metallicnanowire mesh to high temperatures without the present rigid encapsulantaccording to an embodiment of the present invention;

FIG. 4 is a SEM image of a metallic nanowire mesh that has been coatedwith the present encapsulant according to an embodiment of the presentinvention;

FIG. 5 is a SEM image illustrating how an encapsulated metallic nanowiremesh maintains its shape even at temperatures greater than or equal toabout 600° C. according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of an exemplary apparatus used to testresistance of a metallic nanowire mesh sample containing the presentencapsulant according to an embodiment of the present invention;

FIG. 7 is a diagram illustrating resistance as a function of temperaturefor the present encapsulated metallic nanowire mesh, a sample of theencapsulant without nanowires, and a conventional transparent electrodematerial according to an embodiment of the present invention; and

FIG. 8 is a SEM image of a sample of the present metallic nanowire meshwherein palladium plating has been used to coat and fuse the nanowiresin the mesh according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As provided above, metallic nanowires meshes (such as silver (Ag)nanowire meshes) are a good option for use as a transparent electrodematerial since they offer excellent conductivity and allow reasonabletransmission of light at all wavelengths. One drawback however withmetallic nanowires meshes is that at high temperatures the nanowiresundergo structural changes which render films of the materialnon-electrically conductive due to loss of inter-connectivity of thesilver, and thus not suitable for use as an electrode. For instance,some photocatalytic or other optoelectronic devices have operatingtemperatures at or exceeding 600° C. This can cause the structure ofmetallic nanowires in a mesh to change (the nanowires essentiallyball-up) and no longer provide a mesh of interconnected wires.Advantageously, provided herein are techniques for mechanicallystabilizing metallic nanowires by encapsulating them with a coating ofrigid material. The material used for the encapsulant—such as an oxidematerial, is stable and prevents the metallic nanowires from changingstructurally at high operating temperatures.

An overview of the present techniques is now provided by way ofreference to methodology 100 of FIG. 1 which provides a method forforming a mechanically-stabilized metallic nanowire mesh. In step 102, asuspension of metallic nanowires is prepared. By way of example only,the suspension is formed by dispersing the nanowires in a solvent suchas water or isopropyl alcohol.

According to an exemplary embodiment, the metallic nanowires are silvernanowires. Silver nanowires are commercially available, for example,from Sigma-Aldrich®, St. Louis, Mo. The present techniques are howevergenerally applicable to any type of metallic nanowire, such as silvernanowires, nickel nanowires, palladium nanowires, platinum nanowires,gold nanowires, etc.

In step 104, the nanowire suspension is then deposited onto a substrate,forming a nanowire mesh on the substrate. By way of example only, thenanowire suspension can be drop-wise applied to the surface of thesubstrate and permitted to dry. Once the solvent has evaporated, a meshof the nanowires will remain on the substrate.

The term ‘substrate’ as used herein generally refers to any structure onwhich the formation of a metallic nanowire mesh is desirable. Forinstance, as provided above, metallic nanowire meshes are useful astransparent electrode materials. Further, in accordance with the presenttechniques, the metallic nanowire meshes provided herein are thermallystable. Namely, the encapsulant formed on the nanowire mesh (asdescribed below) physically prevents the structure of the nanowires fromchanging during high operating temperatures. Thus, the present metallicnanowire mesh materials are ideal for use in applications requiring bothconductive and thermally stable transparent electrodes. By way ofexample only, one such application is in a high-temperaturephotocatalytic device. See, for example, U.S. patent application Ser.No. 14/456,708 by Gershon et al., entitled “Techniques forPhotocatalytic Hydrogen Generation,” (hereinafter “U.S. patentapplication Ser. No. 14/456,708”), the contents of which areincorporated by reference as if fully set forth herein. In U.S. patentapplication Ser. No. 14/456,708 a hydrogen producing cell is describedthat includes an anode electrode that is transparent and can be porous(i.e., permeable to gasses). The present metallic nanowire meshes are asuitable material for use in forming the anode electrode of thishydrogen producing cell since during operation the environment withinthe cell can reach temperatures of (or exceeding) 600° C. Specifically,since the present metallic nanowires are thermally stabilized (via theencapsulant) one can avoid any potential issues arising from structuralchanges to the nanowires occurring during operation of the cell. In theworst case scenario, without structural stability, the metallicnanowires can ball-up (see example described below) destroying thecontinuity of the mesh and making the electrode overall non-conductive.Thus, in the case where the present techniques are being employed toform a transparent electrode on a device, such as on the hydrogenproducing cell described in U.S. patent application Ser. No. 14/456,708,the term ‘substrate’ as used herein refers to the layer of the device onwhich the transparent electrode is to be formed.

Optionally, in step 106 plating can be used to fuse the junctionsbetween the individual nanowires in the mesh. This optional plating stephelps ensure connectivity between the nanowires, as well as addsrigidity and stability to the overall mesh. Specifically, as-made themetallic nanowire meshes are conductive due to the overlap between thenanowires (where the nanowires contact one another). Therefore, as perstep 104, a network of interconnected nanowires is formed so that thereis electrical connectivity from one end of the mesh to the other.Plating, however, actually fuses the spots together where the nanowirescontact each other. With as-formed meshes, just-touching seems to givereasonable connectivity—but actually coating each of these junctionswith a metal solidifies the joints where they meet and improves therigidity and mechanical stability of the overall mesh.

According to an exemplary embodiment, the metal used to fuse the jointsbetween the nanowires is palladium (Pd). Being a metal, palladium is asuitable choice since it can be plated and, once deposited, iselectrically conductive throughout the mesh. For instance,electroplating can be used to deposit a coating of palladium onto themetallic nanowire mesh. The coating will cover over the junction betweennanowires, physically fusing the nanowires together. By way of exampleonly, a voltage/current can be applied in a standard electrochemicalbath containing palladium ions, in which the metallic nanowire mesh isplaced as the ‘working electrode.’ The palladium ions will plate ontothe nanowire mesh.

In step 108, the metallic nanowire mesh (optionally with a palladiumcoating) is then encapsulated in a rigid, thermally stable material.According to an exemplary embodiment, the rigid, thermally stablematerial is a metal oxide. A metal oxide encapsulant may be formed onthe metallic nanowire mesh in a number of different ways. By way ofexample only, with a solution-based approach a metal oxide precursorsolution can be prepared and then sprayed onto the metallic nanowiremesh. Some heat may optionally be applied (e.g., a temperature of lessthan or equal to about 350° C.) to the sample to drive off solventand/or form a better quality oxide material. This process results in acoating of the metal oxide on the mesh. Alternatively, the metal oxidemay be deposited onto the metallic nanowire mesh by evaporation,sputtering, etc.

Suitable metal oxides include, but are not limited to, titanium oxide(TiO₂), tin oxide (SnO₂), gallium oxide (Ga₂O₃), and doped forms ofthese materials. Doping serves to increase the electrical conductivityof the metal oxides which can be advantageous when, for example, thepresent metallic nanowire mesh is being used as transparent electrode.TiO₂, SnO₂, and Ga₂O₃, can all be made conductive through doping. Forinstance, as is known in the art, suitable dopants for SnO₂ include butare not limited to, fluorine (F). Fluorine-doped SnO₂ is conductive,while SnO₂ itself acts as an insulator. Suitable dopants for Ga₂O₃include but are not limited to, niobium (Nb), and suitable dopants forTiO₂ include but are not limited to, iron (Fe) and nickel (Ni).

In one exemplary embodiment, the metal oxide is titanium oxide. Asprovided above, a solution-based deposition technique is one possibleway to form the metal oxide encapsulant on the metallic nanowire mesh,wherein a metal oxide precursor solution is prepared and sprayed ontothe metallic nanowire mesh. According to an exemplary embodiment, theprecursor solution is prepared by mixing 1 milliliter (mL) titaniumisopropoxide+1 mL acetylacetonate+8 mL ethanol, and is then sprayed ontothe nanowire mesh with the substrate held at a temperature of from about150° C. to about 200° C., and ranges therebetween. Titaniumisopropoxide:

is commonly used as a precursor in titanium oxide precursor solutions.See, for example, Oja et al., “Properties of TiO₂ Films Prepared by theSpray Pyrolysis Method,” Solid State Phenomena, vols. 99-100, pgs.259-264 (July 2004), the contents of which are incorporated by referenceas if fully set forth herein (wherein a precursor solution for spraypyrolysis of titanium oxide thin films contained titanium (IV)isopropoxide (TTIP) as a titanium source, acetylacetone (AcAc) as astabilizer and ethanol as a solvent). See also Parra et al., “ReactionPathway to the Synthesis of Anatase via the Chemical Modification ofTitanium Isopropoxide with Acetic Acid,” Chem. Mater., 20, pgs. 143-150(2008), the contents of which are incorporated by reference as if fullyset forth herein.

Metal oxides, such as titanium oxide, are thermally stable undersemiconductor device operating temperatures, including the elevatedtemperatures encountered in a hydrogen producing cell as referencedabove. Thus, during operation, when temperatures reach or exceed 600°C., the present encapsulant is surrounding the metallic nanowires andthus physically preventing the microstructure of the nanowires fromchanging—i.e., the encapsulant maintains the shape of the nanowires.Specifically, when elevated temperatures are encountered, the metallicnanowires tend to ball up (see, for example, FIG. 3—described below).However, if the nanowires are contained within the present encapsulantthey are physically prevented from balling-up, and forced to maintaintheir shape.

FIG. 2 is a scanning electron micrograph (SEM) image of a metallic (inthis case silver) nanowire mesh that may be prepared, for example, inaccordance with steps 102 and 104 of methodology 100 (described above).As shown in FIG. 2, in the as-formed mesh the individual nanowires aretouching one another providing continuity from one end of the mesh tothe other. In addition, as provided above, plating can optionally beused to fuse the junctions between the nanowires to add furthermechanical stability and rigidity to the nanowire mesh.

FIG. 3 is a SEM image of what happens to a metallic (in this casesilver) nanowire mesh when it is subject to high temperatures withoutthe present rigid encapsulant. FIG. 3 illustrates the problem solved bythe present techniques. Namely, by way of comparison with the image ofthe nanowire mesh shown in FIG. 2, as shown in FIG. 3 when subject tohigh temperatures the structure of the nanowires changes (i.e., thenanowires ball-up). This structural change is permanent and, as aresult, connectivity between the nanowires is lost and the film is nolonger conductive. In this example, the as-formed metallic (in this casesilver) nanowire mesh was heated at a temperature of 600° C. for onehour with steam (to simulate the conditions typically encountered in theabove-referenced hydrogen producing cell).

FIG. 4 is a SEM image of a metallic (in this case silver) nanowire meshthat has been coated with the present encapsulant (in this case atitanium oxide encapsulant) using the above-described process. The meshhas the same basic as-formed configuration as shown in FIG. 2, where thenanowires in the mesh contact one another providing connectivity fromone end of the mesh to the other. Thus, the process for preparing andapplying the encapsulant does not change the overall arrangement of themesh. Advantageously, as shown in FIG. 5, when the encapsulated metallicnanowire mesh is subjected to temperatures of about 600° C. for one hourwith steam (the same conditions which caused the microstructural changesin the uncoated nanowires as shown in FIG. 3), the encapsulatednanowires maintain their shape. Specifically, a comparison of the imagesshown in FIG. 4 and in FIG. 5 reveal that the structure of the nanowiremesh has not changed despite prolonged heating at elevated temperatures.The only difference in the samples shown in FIG. 3 and in FIG. 5 is thatthe present encapsulant has been coated on the mesh. This demonstratesthat the encapsulant is responsible for maintaining the shape of thenanowires in the mesh.

Experiments were also conducted to measure the resistance of varioussamples both with and without the present encapsulant in place. FIG. 6is a schematic diagram of an exemplary apparatus used to test resistanceof the samples. As shown in FIG. 6, the testing apparatus includes aglass substrate upon which each sample was placed. Platinum meshelectrodes attached to platinum wires were placed in contact withopposite ends of the sample and, after a heat treatment, resistanceacross each sample was measured. In this example, the samples testedincluded a silver nanowire mesh stabilized with the present encapsulant(see, for example, FIG. 4), plain titanium oxide (with no nanowires),and a commercial transparent electrode material—ITO. All samples wereprepared as thin films on glass substrates. The titanium oxide samplewas included in order to show that it is the encapsulated nanowiresthemselves (rather than the encapsulant) that remain conductive afterthe high temperature heat treatment.

A separate testing apparatus was used for each sample. Each testingapparatus with its respective sample was placed in a furnace with steam,and the resistance of the sample was measured as a function oftemperature.

The results of the resistance tests are shown in FIG. 7. FIG. 7 is adiagram illustrating resistance of the samples as a function oftemperature (T). In FIG. 7, temperature (measured in ° C.) is plotted onthe x-axis and resistance (measured in Ohms) is plotted on the y-axis.As the plot in FIG. 7 illustrates, the resistance of the nanowire meshwith the present encapsulant changed only slightly as temperatures offrom about 400° C. to about 600° C. were reached. This trend is similarto what happened with the commercial ITO sample. The resistance of thetitanium oxide sample (without any nanowires) was too high to measure atroom temperature, and decreased only slightly at the elevatedtemperatures. This illustrates that the conductivity of the encapsulatednanowires mesh sample detected after the heating is due to the nanowiresthemselves, not the metal oxide (in this case titanium oxide)encapsulant. Namely, this shows that, by way of the encapsulant, theshape of the nanowires is maintained throughout the heating processthereby maintaining the integrity of the mesh. To look at it in anotherway, this result proves that the nanowires are not balling-up within theencapsulant, because in that case based on the resistance of theencapsulant the resistance of the sample would be very high.

Based on this sample testing, it can be concluded that the presentencapsulated metallic nanowire mesh remains conductive at temperaturesgreater than or equal to 600° C., and that the microstructure of theencapsulated nanowires does not degrade after one hour in steam at 600°C. By comparison, the resistance of a similar metallic mesh nanowire butwithout encapsulation would reach infinity at such high temperatures dueto the overall loss of conductivity resulting from changes in thenanowire structure and breakdown of the mesh. The above examples alsoshow that the present encapsulated/stabilized metallic nanowire mesh isat least as conductive as commercial ITO at all temperatures tested.

As provided above, the present metallic nanowire meshes can be furthermechanically stabilized through the use of metal plating which serves tofuse the junctions between the nanowires. As described above, thisoptional plating step can be carried out by plating the whole mesh witha metal such as palladium, prior to placing the encapsulant. By fusingthe nanowires together, their connectivity is increased and the mesh ismade overall more rigid and robust.

FIG. 8 is a SEM image of a sample of the present metallic nanowire meshwherein palladium plating has been used to coat and fuse the nanowiresin the mesh. Specifically, the nanowires shown perpendicular to oneanother in the center of FIG. 8 have been fused together by way of thepalladium coating plated on to the mesh. By fusing the nanowirestogether, the overall rigidity of the mesh is increased. Specifically,in an as-formed mesh if these two nanowires were in contact, but notfused to one another, then the mesh would be less rigid. However, by wayof the plating process, a network is formed where the nanowires arephysically anchored to one another.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A mechanically-stabilized metallic nanowire mesh, comprising: a metallic nanowire mesh on a substrate; and a metal oxide coating on the metallic nanowire mesh that encapsulates the metallic nanowire mesh to mechanically-stabilize the metallic nanowire mesh which permits the metallic nanowire mesh to remain conductive at temperatures greater than or equal to about 600° C.
 2. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metallic nanowire mesh comprises silver, nickel, palladium, platinum, or gold nanowires.
 3. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises a material selected from the group consisting of: titanium oxide, tin oxide, gallium oxide, and doped forms thereof.
 4. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises titanium oxide.
 5. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises fluorine-doped tin oxide.
 6. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises niobium-doped gallium oxide.
 7. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises iron-doped titanium oxide.
 8. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the metal oxide comprises nickel-doped titanium oxide.
 9. The mechanically-stabilized metallic nanowire mesh of claim 1, further comprising: a metal fusing individual metallic nanowires in the metallic nanowire mesh together.
 10. The mechanically-stabilized metallic nanowire mesh of claim 9, wherein the metal comprises palladium.
 11. The mechanically-stabilized metallic nanowire mesh of claim 9, wherein the metal is an electroplated metal covering the metallic nanowire mesh.
 12. The mechanically-stabilized metallic nanowire mesh of claim 1, further comprising: electrodes in contact with opposite ends of the metallic nanowire mesh coated with the metal oxide.
 13. The mechanically-stabilized metallic nanowire mesh of claim 1, wherein the electrodes comprise a platinum mesh.
 14. A mechanically-stabilized metallic nanowire mesh, comprising: a metallic nanowire mesh on a substrate; a metal fusing individual metallic nanowires in the metallic nanowire mesh together; and a metal oxide coating on the metallic nanowire mesh that encapsulates the metallic nanowire mesh to mechanically-stabilize the metallic nanowire mesh which permits the metallic nanowire mesh to remain conductive at temperatures greater than or equal to about 600° C.
 15. The mechanically-stabilized metallic nanowire mesh of claim 14, wherein the metallic nanowire mesh comprises silver, nickel, palladium, platinum, or gold nanowires.
 16. The mechanically-stabilized metallic nanowire mesh of claim 14, wherein the metal oxide comprises a material selected from the group consisting of: titanium oxide, tin oxide, gallium oxide, and doped forms thereof.
 17. The mechanically-stabilized metallic nanowire mesh of claim 14, wherein the metal oxide comprises titanium oxide.
 18. The mechanically-stabilized metallic nanowire mesh of claim 14, wherein the metal comprises palladium.
 19. The mechanically-stabilized metallic nanowire mesh of claim 14, wherein the metal is an electroplated metal covering the metallic nanowire mesh.
 20. The mechanically-stabilized metallic nanowire mesh of claim 14, further comprising: platinum mesh electrodes in contact with opposite ends of the metallic nanowire mesh coated with the metal oxide. 